The present invention relates to epitaxial reactors, i.e. apparatuses in which substrates or wafers of various materials are covered through epitaxial growth by means of vapor phase chemical deposition, as disclosed for example by the U.S. Pat. No. 3,749,383 Voigt et al. The invention relates in particular to epitaxial deposition of monocrystal silicon on silicon substrates or wafers for the fabrication of semiconductor devices. Epitaxial growth on silicon wafers or substrates is a process which takes place at temperatures between 900.degree. C. and 1250.degree. C. in flowing hydrogen and other gases (e.g. SiCl.sub.4 or SiHCl.sub.3) using a graphite susceptor covered with gas-proof coatings which do not react with them. The coatings typically consist of silicon carbide on which are set the substrates of wafers. Growth takes place on the face of the substrates in contact with the gas.
The susceptor has a truncated pyramid shape and is enclosed in a slightly larger quartz bell jar transparent to wavelengths of about 1 micron in which the gases may flow. The susceptor of the kind disclosed in U.S. Pat. No. 3,749,383 rotates around its vertical axis to obtain a uniform temperature and distribution of the reaction gases. To bring the susceptor to the required temperature and hold it there during the growth process (20-90 minutes) electromagnetic induction is used with a specifical generator and an oscillatory circuit including capacitors and an inductor provided by a coiled electrical conductor hereinafter called a coil, would externally around the quartz bell jar and coaxial with the susceptor.
The crystallographic defects of the product (typically silicon wafers) are related to the transverse temperature gradient from the back to the front of the substrate which brings on cup-like warping of the substrate due to the expansion differential between the front and the back of the substrate, inducing stresses in the crystal lattice which in some points exceed the elasticity limit. This causes slipping of the crystallographic planes. The lines of slippage and dislocation remain after cooling and constitute zones where the proper working of the semiconductor is distorted, lowering the yield of the product. With an induction heating system the temperature gradient cannot be completely eliminated but, to improve the product quality it is advisable to minimize the gradient by creating an isothermal optical cavity, i.e. reflecting on the susceptor and on the wafers as much as possible of the energy radiated by them.
There were in the prior art some attempts to use substantially cylindrical mirrors in order to reflect back on a susceptor, or similar body, heated by induction, the thermal energy irradiated by it (see for example the German published patent application No. 1,924,997 and U.S. Pat. No. 3,699,298. However, the reflecting lining 5 of German published patent application No. 1,924,997, working as a short circuited turn inside the induction coil 3, dissipates as heat part of the radio frequency power supplied by the coil 3 and, not having any efficient heat sink to dissipate the so formed heat, can properly work only at very limited power levels (at most of the order of one or two kilowatts). Such a reflection would be destroyed, through overheating and melting, by power levels between 70 and 150 kilowatts, as provided by the present invention.
The Faraday shield 42 mentioned in U.S. Pat. No. 3,699,298 works as a short circuited turn also and can stand the dissipated high power level owing to efficient water cooling but must face all the problems connected with an excessive power dissipation typical of the prior art.
U.S. Pat. No. 4,284,867 partially solves this problem by using a half cylindrical mirror properly divided into two separated quadrants to avoid the problem of the radio frequency energy dissipation through induction in the mirrors, but, unfortunately the mirror surrounds just the half of a susceptor, introducing some dissymmetry in the reflected thermal energy, thereby enhancing the temperature gradients to be most avoided.
It should be noted that for a larger diameter of the silicon wafer there is a correspondly greater sensitivity to crystallographic defects which is fatal for the yield of the product. At the same time the trend in the semiconductor industry is toward ever larger diameters (from a diameter of 75 mm (3") in 1979 to a diameter of 125 mm (5") in 1983 with an expected diameter between 150 mm and 200 mm (between 6" and 8") in the nineteen-eighties).
The main object of the present invention is to provide a system capable of reflecting the thermal energy radiated by the susceptor on to the susceptor itself and on the substrates without substantially absorbing electromagnetic energy from the inductor, with a resulting lower absorption of electrical power under steady operating conditions, and to reduce the transverse temperature gradient on the silicon substrates.
These objects are achieved with an epitaxial reactor of the type comprising at least one quartz bell jar in which is mounted a graphite susceptor covered with a coating of appropriate known materials (such as silicon carbide) on which are accommodated the substrates of wafers to be treated, said susceptor, being able to rotate around its vertical axis, said bell jar being fed with a mixture of gases of appropriate known composition (e.g. hydrogen saturated with SiCl.sub.4 or SiHCl.sub.3). The susceptor is heated by electromagnetic induction produced by a generator and an oscillatory circuit comprising capacitors and an inductor consisting of a coil or solenoid, wound externally around the quartz bell jar and coaxial with the susceptor, wherein the generator consists of a static converter operating at medium frequency (between 1 kHz and 20 kHz), and around said quartz bell jar is arranged a screen, having a cylindrical shape, capable of returning to the susceptor the energy radiated by the same. The screen is made of material capable of reflecting the wavelengths of the peak emissivity of the susceptor and is capable of the lowest coupling with the electromagnetic field of the inductor depending on the frequency of oscillation in order to have a minimum power dissipation therein.
It should be noted that among the many epitaxial reactors which historically became popular in the semiconductor industry, one that distinguishes itself by excellent features is disclosed in U.S. Pat. No. 4,579,080. That apparatus uses a susceptor having a multi-face prismatic or truncated pyramidal shape, enclosed in an insulating bell jar non-reactive in respect of the chemical vapors inside said bell jar, the bell jar being covered by a reflective layer, such as a metallic layer, to reflect the thermal energy irradiated by said susceptor during operation, in order to reduce the power consumption and to render the temperature of said susceptor uniform. The bell jar is surrounded by a cooling liner cooled by a liquid, such as water, to hold the walls of said bell jar to such a temperature as to prevent a substantial deposition on said walls of material potentially contaminating to the substrates treated in the reactor. As further disclosed in the patent the bell jar is surrounded by an inductor or coil energized by medium-frequency alternating current (between 1 and 15 kHz) to provide power to heat said susceptor, the inductor or coil being formed by a conductor wound in many turns properly spaced by provide to said susceptor a power variable from a point to another, as particularly specified at column 12, lines 59, 67 of the patent. If the local power in the susceptor must be changed, the spacing between adjacent turns must be varied through control means controlling the turn spacing, which control means have various mechanical and spacing range problems associated with the dimensions of the conductors. Moreover, where the turn spacing is large, the conductors forming the coil will lie in planes which are no longer perpendicular in respect of the susceptor axis and, as a consequence, the electrical current which is induced parallel to the turns results in local heating producing the same substantial thermal gradient. Owing to the susceptor rotation, the thermal gradient produces a local pulsating heating, thereby enhancing those thermal gradients which are sought to be reduced, with the consequent production of crystal defects or flaws in the substrates or wafers.
Further the susceptor has thickens or projections in some regions, such as regions around pockets or indentations housing the substrates or wafer to be heated (see column 8, lines 50-55), and has thinning at some other regions as the dihedral corners and upper and lower regions (see column 13, line 66 to column 14, lines 14), for the purpose of distributing the heat according to desired patterns.
However, such patterns required a tedious and time-consuming empirical process to determine the shape giving the best thermal results and producing more uniform temperature profiles across the faces of the sidewalls of the susceptor.
Lastly, it is asserted in the patent that, owing to the heat irradiated by the susceptor and to the heat produced by induced currents within the reflecting metallic layer lining the quartz bell jar, it is very difficult to obtain efficient air cooling and, as a consequence, it is preferred to use water cooling provided by a water liner surrounding said bell jar (see column 11, line 57 to column 12, line 17).